U-Th Zircon Dating by Laser Ablation Single Collector Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)

Zircon crystals in the age range of ca. 10–300 ka can be dated by 230Th/238U (U-Th) disequilibrium methods because of the strong fractionation between Th and U during crystallisation of zircon from melts. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of nine commonly used reference zircons (at secular equilibrium) and a synthetic zircon indicates that corrections for abundance sensitivity and dizirconium trioxide molecular ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon. When corrected for abundance sensitivity and interferences, mean activity ratios of (230Th)/(238U) for nine reference zircons analysed on five different days averaged 0.995 ± 0.023 (95% confidence weighted by data-point uncertainty only, MSWD = 1.6; n = 9), consistent with their U-Pb ages > 4 Ma that imply equilibrium for all intermediate daughter isotopes (including 230Th) within the 238U decay chain. U-Th zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th are potentially unreliable. To validate the applicability of LA-ICP-MS to this dating method, we acquired data from three late Quaternary volcanic units: the 41 ka Campanian Ignimbrite (plutonic clasts), the 161 ka Kos Plateau Tuff (juvenile clasts) and the 12 ka Puy de Dôme trachyte lava (all eruption ages by Ar/Ar, with zircon U-Th ages being of equal or slightly older). A comparison of the corrected LA-ICP-MS results with previously published secondary ion mass spectrometry (SIMS) data for these rocks shows comparable ages with equivalent precision for LA-ICP-MS and SIMS, but much shorter analysis durations (~ 2 min vs. ~ 15 min) per spot with LA-ICP-MS and much simpler sample preparation. Previously undated zircons from the Yali eruption (Kos-Nisyros volcanic centre, Greece) were analysed using this method. This yielded a large age spread (~ 45 to > 300 ka), suggesting significant antecryst recycling. The youngest zircon age (~ 45 ± 10 ka) provides a reasonable maximum estimate for the eruption age, in agreement with the previously published age using oxygen isotope stratigraphy (~ 31 ka).